Defibrillation is the process of applying a strong electrical "countershock" to a patient's heart in an effort to convert an excessively fast heart rhythm--i.e., in order to slow and correct the heart rhythms and thereby allow the heart to pump more blood. A defibrillator is a device which applies such "countershocks." For example, "external" defibrillators produce and store energy for delivery to the heart through a pair of externally attached electrodes, wherein the actual delivered energy is a function of both the stored energy in the defibrillator device and the "transthoracic impedance" of the patient. Thus, for a given stored energy level, the transthoracic impedance determines the amount of current which flows to the myocardium of the patient. In particular, this current flow must be adequate to depolarize a critical mass of myocardium to achieve defibrillation.
The transthoracic impedance consists of two parts: the impedance due to the electrode-electrolyte interface at the connection site to the patient, hereafter referred to as "electrode impedance," and the impedance due to patient tissue. Impedance itself is comprised of two parts: resistance, which is constant, and reactance, which is frequency dependent. Historically, transthoracic and electrode impedance have been estimated using a method first proposed by Geddes in 1975. With this method, an "apparent" impedance is calculated as a ratio of peak voltage to peak current obtained when defibrillation energy is applied across a patient's chest, or, for purposes of testing patient electrodes, to face-to-face electrodes. As a variation of this method, Geddes showed that in dogs the ratio of peak voltage to peak current obtained from a high frequency (approximately 30 kHz) sinusoidal current approximates the ratio obtained from application of defibrillation energy.
Kerber later "validated" this approximation in 19 human subjects using a 31 kHz square wave voltage and a Hewlett Packard 78660 defibrillator, obtaining a mean impedance of 78.1.+-.19.4.OMEGA.. In a paper describing these tests, Kerber used the description of "actual" impedance. However, this "actual" impedance, as calculated by Geddes and Kerber, assumes this impedance is a pure resistance, whereas the true impedance is not purely resistive, since it varies with frequency. Nevertheless, this method of impedance estimation has been used consistently for the past two decades to estimate transthoracic and electrode impedance. For example, Hewlett Packard has incorporated the high frequency ratio measurement in its defibrillators in the operation of a patient contact indicator, as described in U.S. Pat. No. 4,840,177.
Thus, it has been assumed since 1975 that the "apparent" impedance accurately estimates the true resistance. However, a simple experiment can be used to determine the accuracy of the apparent impedance ratio calculation, based on connecting the positive and negative outputs of a recently calibrated Hewlett Packard 43110 defibrillator directly to various resistors, each rated for 25 W, placed in series with a 50.6 .mu.F capacitor, rated for 5 kV. For each resistor value, the apparent impedance, as estimated according to the Geddes/Kerber methodology, was measured from the mean of three measurements as:
______________________________________ True Resistance (.OMEGA.) "Apparent" Impedance (.OMEGA.) ______________________________________ 50.4 105 502 90 999 105 ______________________________________
As can be observed from these figures, regardless of the true resistance value taken over a large resistive range, the measured apparent impedance is approximately constant. In other words, the apparent impedance method is predisposed to estimating resistance values of approximately 100.OMEGA., even if the true resistance is an order of magnitude higher. In fact, Geddes himself has acknowledged the shortcomings of his measurement, stating, in a recent writing, that "in defibrillation studies many authors use the term impedance [Geddes' italics] to mean the ratio of peak voltage to peak current for the defibrillating current pulse, regardless of waveform. Although this practice violates the strictest definition of impedance, the ratio so obtained informs about the quality of the conducting pathway."
It is believed, however, that the Geddes/Kerber "apparent" impedance ratio may not, in reality, be providing sufficient information about the "quality of the conducting pathway," especially given the notable flaws in the underlying assumptions of this methodology pointed out by the above experiment. A significant need therefore exists for determining true electrode and transthoracic impedance. In particular, the true impedance estimate should consider both the resistance and reactance components of impedance, as opposed to a "lumped" resistance approach of Geddes/Kerber.